The invention relates to a line synchronizing circuit for a picture display device, comprising a control loop for controlling the frequency and/or the phase of a line oscillator, having means for applying an incoming pulse-shaped line synchronizing signal and a reference signal generated by the oscillator to a phase discriminator for determining the phase difference between the signals applied thereto and having a loop filter for smoothing the phase discriminator output signal and for applying the smoothed control signal obtained to the oscillator for the control thereof, said control loop being such that during operation, in the target state thereof, the centre instant of an edge of the reference signal substantially coincides with the center instant between a leading edge and the subsequent trailing edge of a line synchronizing pulse, the line synchronizing circuit also comprising a coincidence detector for detecting a phase difference between an incoming line synchronizing pulse and an oscillator pulse which difference is less than a predetermined value and for changing over the mode of operation of the control loop.
Such a line synchronizing circuit is disclosed in U.S. Pat. No. 4,214,260. In this known circuit, a number of change-over operations are effected by means of the coincidence detector for accellerating pull-in of the control loop when there is no coincidence and for increasing the noise insensitivity when there is coincidence, that is to say in the pulled-in state.
Line synchronizing circuits which, as the circuit mentioned above, control at the center instant of the line synchronizing pulses, have the disadvantage that a phase error occurs at the commencement of the field period, which error is caused by the fact that the duration of the synchronizing pulses occurring during the field blanking period is, different from the duration of the line synchronizing pulses occurring during the field trace period. As a result thereof, the content of the phase discriminator output signal differs from its target value, which results in an error in the control which has not yet been corrected at the end of the field blanking period. As a result thereof, vertical lines are displayed as curved lines at the top of the picture display screen. This error can be corrected, several methods to do so are known. It is, for example, possible to render the control loop inoperative during the field blanking period. This has however the disadvantage that pull-in is delayed.